Request confirmation on file overwrite

Once #4 (closed) will be implemented, accidentally overwriting existing files may become more frequent, especially .mat files, as the user will have to type the filename as a replacement for the default .labels filename, possibly without a timestamp in the name.

Proposal: If a file already exists at the designated location, inform the user and request confirmation with a dialog box.

EDIT: note that, as of now, it is possible to save labels in the json format but with trx.mat as a filename, for example.